One of the key advantages of double-sided SMT is the potential of populating both sides of the PCBs with surface mount components. Double-sided SMT provides a two-fold increase in potential interconnection density and product density. However, there are many process problems which limit the use of double-sided SMT.
Currently, double-sided SMT technology has difficulties with the process tooling necessary to print, place components and reflow with high reliability. Obviously, the second side of a PCB is in essence a different board. A separate stencil is needed, a different pick and place setup is required and a separate reflow profile is necessary.
Unlike a single-sided board, the second side of an SMT board must be processed with a finished assembly affixed to the first side. Associated problems include component damage, warpage from reflow sagging and lack of support during printing and reflow. Further, double-sided SMT results in decisions regarding which side to print first, where the board support will be, warpage after first side reflow, more severe warpage at second side reflow, i.e. more weight. Further, second side reflow can need more heat time or higher set temperatures.
There are fixtures currently available to hold PCBs, i.e. PCB workboard holders and flow solder carriers, which are adjustable along the X, Y axes. However, these currently available fixtures are typically large and cumbersome and in most instances are custom designed for specific PCB applications. Further, they are generally characterized by their lack of total peripheral support of the PCB and lack of point support within the outline of the PCB to prevent flexing. Further, prior art fixtures generally use off-the-shelf components with invariably protruding features on their periphery and extending above the plane of the PCB. There still exists the combined problems of board support, flat profile and adjustability.
The present invention embodies a universal adjustable rectangular fixture for PCBs. The invention comprises a kit of structural members which can be assembled to form a fixture of substantially unlimited dimensions which correspondingly can support variously dimensioned PCBs. The fixture of the invention avoids the necessity of custom designing fixtures for individual PCBs and for PCB processing machines. The fixture of the invention when adjusting inwardly or outwardly along the X and Y axis always maintains a square or rectangular perimeter without extending projections.
The kit, in one embodiment, comprises at least four corner frames at least one of which has a location hole and/or positioning pin, a plurality of adjusting bars which in use are slidably received in the corner frames and means to lock the members rigidly together as a fixture. In a preferred embodiment, the kit comprises at least two side frames and at least one vertical support block adapted to engage a support bar. When the side frames are used the support bars are slidably received therein.
In the present invention, there is adjustability of the fixture not only in the X, Y axes but also in an r-.theta. axis for tooling pins for PCB alignment. Further, there is adjustable vertical support for the PCB along the X, Y and Z axes. The fixtures have a flush top surface, either coplanar with the top surface of the PCB or recessed with reference to the top surface of the PCB. There are no extending features beyond the rectangular perimeter so that the fixture itself can easily feed into processing equipment. Further, with the fixture of the invention, the PCB can remain in situ during the printing of the solder pattern, the placement of the components and the reflow step.
In a preferred embodiment, the fixture of the invention is adjustable along X, Y axes, it has a flat receding profile, a movable locator pin(s) array, an adjustable center vertical support(s) along X, Y and Z axes, high-rigidity, light-weight and high heat absorption with low thermal mass.
The fixture of the invention can be passed from the printing stage through component placement and into oven processing. The invention eliminates warpage after first side heating which typically causes problems with the second side printing and placement of parts. The PCBs in the fixture are vertically supported "raised" to prevent sagging and components originally placed in the first side are protected from being contacted and damaged. Preferably, the board has point supports at least every four square inches to prevent flexing while the PCB/fixture is being printed.
During reflow, on the second side, the board tends to warp more severely than during the first side processing, because of the additional weight of having components on both sides of the board. The fixture in the invention supports such an assembly. Handling of any SMT board after printing and placement is a challenge since the parts, in the printed pattern are close to the edge of the board. If the board is dropped or if parts are touched or the print is smeared, it can require expensive and time consuming rework. A fixture which moves the handling points away from the board outline is important.
The fixture of the invention provides a continuous protected PCB perimeter, such that the fixture extends the perimeter of the board so that the components no longer sit close to the handling edges. Further, at least fifty percent (50%) edge support and multiple adjustable vertical point support in three dimensions is provided. The structure has a profile equal to or less than the board such that it does not interfere with the application of a solder pattern thereto while in the fixture. There is an overlapping slot length/hole spacing pattern which maximizes rigidity and hole alignment. In an alternative embodiment, the fixture includes a rotatable lip which swings upwardly and outwardly to be engaged by edge grabbing conveyor systems.